For example, with reference to attached FIG. 1, in a CZ method-type single crystal pulling apparatus for growing and raising a single crystal silicon ingot (9) from a polycrystal silicon melt (10), a silicon seed crystal (12) fixed at the lower end of a pull wire (11) (or, alternatively, supported by a pull shaft) is dipped slightly in the silicon melt contained in a quartz crucible (4) provided in a hermetical chamber (2), and is pulled up at a rate while being turned about its axis by means of the pull wire, whereby a growing silicon single crystal rod is raised.
When a material having a high volatility and a small segregation coefficient, such as Sb, is added to the polycrystal melt in the crucible as a dopant, or an impurity, for the purpose of obtaining a doped or impurity semiconductor, e.g. Sb-doped single crystal, an inconvenience is experienced as the single crystal is pulled up and the liquid level of the polycrystal melt lowers in the crucible: that is, since the segregation coefficient of the dopant (Sb) is small, its tendency of entering the growing single crystal is small, and consequently the dopant concentration of the polycrystal melt in the crucible gradually increases as the pulling operation proceeds, so long as the pressure in the chamber is maintained at a constant value (e.g., 30 millibar, as shown by the broken straight line A in FIG. 2) so as to fix the evaporation rate of the dopant throughout the pulling operation.
There exists a relation between the specific resistance of a single crystal and the Sb concentration thereof which is typically represented by the graph of FIG. 4. As the Sb concentration increases, the specific resistance of the single crystal decreases along a curve resembling a hyperbolic curve.
Therefore, as described above, as the single crystal grows longer causing the dopant concentration to increase, that is, as the proportion of the length of the grown crystal to the aimed final length thereof increases (abscissa, %), the specific resistance drops along the broken line a of FIG. 3, and eventually may become lower than the minimum allowable value. Incidentally, in FIG. 3, plotted with a symbol is the actually measured specific resistance.